Fiber optic hydrophone sensors and uses thereof

ABSTRACT

Disclosed is detecting changes in pressure in a medium, with an optical fiber having a core diameter at an immersion surface contact of the fiber of less than 10 μm; a layer of material deposited on said end of the fiber, the material being of a thickness of from about 2 nm to about 10 nm. Also disclosed is detecting pressure waves in a medium comprising: contacting the medium with a fiber optic, the fiber integrated with a light source and a detector, the fiber optic having a diameter of less than 10 μm at an immersion surface contact of the fiber; providing a thin layer of material on the immersion surface contact, wherein said thin layer of material is of a thickness in a range of from about 2 nm to about 10 nm; and detecting Fresnel back reflections from the immersion end of the fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. application Ser. No. 13/150,906(allowed on Oct. 23, 2013), filed Jun. 1, 2011. That application claimsthe benefit of U.S. provisional application Ser. No. 61/350,422, filedon Jun. 1, 2010. All of the foregoing applications are incorporated byreference herein in their entireties for any and all purposes.

STATEMENT OF GOVERNMENT INTERESTS

At least a portion of the work leading to the disclosed inventions wasmade using federal support by the National Institute of Health's NIBIB(National Institute of Biomedical Imaging and Bioengineering) undergrant #R01 EB007117. The government has certain rights in the disclosedinventions.

TECHNICAL FIELD

The disclosed inventions are in the field of devices for detectingpressure amplitude.

BACKGROUND

Hydrophone devices are capable of detecting pressure amplitude inimmersion media such as liquids or gases and in solids as well. Severaldifferent types of hydrophone devices are known in the art. Newapplications for sensing and use of pressure amplitude in liquids,solids and gases, including clinical, sonar, and communicationapplications require improved hydrophones for calibration, metrology,medical metrology, elasticity of medium, imaging, detection, therapy,diagnosis, and the like.

One type of hydrophone device known in the art is a piezoelectrichydrophone device. Piezoelectric hydrophones may be used for measurementof large frequency bandwidths; however, problems arise from thegeneration of high temperatures and cavitation effects that are producedby High Intensity Focused Ultrasound (HIFU) fields, High IntensityTherapeutic Ultrasound (HITU) fields, lithotripter fields or the like.These problems generally could lead to device failure due to highpressure amplitudes. Currently, such devices tend to be costly andcumbersome, and tend to have large apertures which can spatially averagecertain acoustic fields. For example, existing hydrophone probes haveaperture diameters on the order of about 500 μm or more which introducesspatial averaging of acoustic fields beyond 3 MHz. This spatialaveraging can lead to errors in detection and faithful reproduction ofthe pressure-time waveform of the measured acoustic wave and result inpoor spatial resolution.

Other acoustic pressure sensors have been proposed as well, including alimited range of fiber optic based pressure sensors that exploitsamplitude variations. There are at least two other broad classificationsbased on the sensing mechanism for these sensors, namely phase modulatedand wavelength modulated pressure sensors. Included in phase modulatedsensors are Mach-Zehnder interferometers, Fabry-Perot resonantstructures and multilayer resonant structures that act asmicrointerferometers. These interoferometric phase schemes however, aresubject to phase fluctuation which may result in higher amplitude noiseof the sensor signal. Phase fluctuations, temperature drift and otherproblems associated with phase modulated fiber optic hydrophones cancause errors in measurement.

Wavelength modulated phase sensors employing external Bragg's cells,fiber Bragg gratings (FBGs) and distributed Bragg reflectors have alsobeen proposed. These fiber optic hydrophone devices perform acousticsensing based on an acoustically induced change in the wavelength ofoptical signals passing through the given sensor. These wavelengthmodulated sensors are usually distributed along the length of the fiberand have sensing dimensions on the order of a few millimeters. Thetypical range for sensing regions in wavelength modulated sensors is onthe order of about 600 μm to about 3 mm. This large sensing dimensioncauses the sensors to suffer from poor spatial resolution thus limitingthe resolution bandwidth. For this reason, wavelength modulated fiberoptic hydrophones cannot be used in many ultrasound applications.

Thus, what is needed is a novel, high sensitivity sub-micron resolutionrugged hydrophone probe that would be able to characterize acousticfields in the frequency range up to 100 MHz while minimizing spatialaveraging, phase fluctuations, or both.

SUMMARY

Accordingly, the present invention provides for systems for detectingchanges in pressure in an immersion medium such as a liquid, a gas, or asolid, the system comprising: an optical fiber, wherein said opticalfiber has a core diameter at an immersion surface contact of the fiberof less than 10 μm; and a layer of material deposited on said end of thefiber, wherein said layer of material has a thickness of from about 2 nmto about 10 nm. In another embodiment, the fiber may have a diameter ofless than about 20 um, and the diameter may be selected based on thefrequency of the ultra sound.

The present invention also provides for methods for detecting changes inpressure in an immersion medium, such as a liquid or gas, or in a solid(collectively media or medium), the method comprising: contacting themedium with a fiber optic, wherein said fiber optic integrated with alight source and a detector, and wherein said fiber optic has a diameterof less than 10 μm at an immersion surface contact of the fiber;providing a thin layer of material on the immersion surface contact,wherein said thin layer of material is of a thickness in a range of fromabout 2 nm to about 10 nm; and detecting Fresnel back reflections fromthe immersion end of the fiber.

Also provided are methods of making a device for detecting acousticwaves in an immersion medium such as a liquid, gas, or solid,comprising: providing an optical fiber having a core diameter at animmersion surface contact of less than about 10 μm; and depositing onsaid immersion contact surface of the fiber a layer of material having athickness of from about 2 nm to about 10 nm.

The general description and the following detailed description areexemplary and explanatory only and are not restrictive of the invention,as defined in the appended claims. Other aspects of the presentinvention will be apparent to those skilled in the art in view of thedetailed description of the invention as provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The summary, as well as the following detailed description, is furtherunderstood when read in conjunction with the appended drawings. For thepurpose of illustrating the invention, there are shown in the drawingsexemplary embodiments of the invention; however, the invention is notlimited to the specific methods, compositions, and devices disclosed. Inaddition, the drawings are not necessarily drawn to scale. In thedrawings:

FIG. 1 depicts an example of a prior art fiber optic hydrophone.

FIG. 2 depicts an example embodiment of an experimental setup of thepresent invention.

FIG. 3 depicts examples of fiber optic hydrophone immersion surfacecontact geometries.

FIGS. 4(a), (b), (c), (d), (e) and (f) depict a series of immersionsurface contacts and different associated geometries. FIGS. 4(a)-(b)depict a fiber with a straight cleaved immersion end, FIGS. 4(c)-(d)depict a fiber with a cylindrically etched immersion end, and FIGS.4(e)-(f) depict a fiber with a tapered immersion end.

FIG. 5 depicts a previous model suggesting that there was no improvementin hydrophone sensitivity for a coating thicknesses for gold of lessthan about 30 nm.

FIG. 6 depicts surprisingly good experimental results for sensitivity ofa fiber optic hydrophone with thin layers of gold on the immersion endof the fiber.

FIGS. 7(a) and (b) depict the sensitivity effects of core diameter in anexample embodiment of a cylindrically etched fiber and the sensitivityeffects of taper angle in an example embodiment. The units on the leftside of the graph would show a relative improvement in sensitivity in dBand exists as an example.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention may be understood more readily by reference to thefollowing detailed description taken in connection with the accompanyingfigures and examples, which form a part of this disclosure. It is to beunderstood that this invention is not limited to the specific devices,methods, applications, conditions or parameters described and/or shownherein, and that the terminology used herein is for the purpose ofdescribing particular embodiments by way of example only and is notintended to be limiting of the claimed invention. Also, as used in thespecification including the appended claims, the singular forms “a,”“an,” and “the” include the plural, and reference to a particularnumerical value includes at least that particular value, unless thecontext clearly dictates otherwise. The term “plurality”, as usedherein, means more than one. When a range of values is expressed,another embodiment includes from the one particular value and/or to theother particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another embodiment. All ranges areinclusive and combinable.

It is to be appreciated that certain features of the invention whichare, for clarity, described herein in the context of separateembodiments, may also be provided in combination in a single embodiment.Conversely, various features of the invention that are, for brevity,described in the context of a single embodiment, may also be providedseparately or in any subcombination. Further, reference to values statedin ranges include each and every value within that range.

As used herein, an immersion medium is any liquid, solid or gas capableof transmitting acoustic waves. An immersion end of the fiber optichydrophone is the end of the fiber used in Fresnel back reflection modeand the immersion surface contact is the tip of the immersion end of thefiber having Fresnel back reflections associated therewith.

In an embodiment, an intensity modulated fiber optic hydrophoneoperating in a Fresnel reflective mode is provided with a fiber having adiameter at the immersion end of the fiber of under about 10 μm in orderto characterize a frequency range of from about 0.5 MHz to about 500MHz, where, as a specific example, a frequency range of up to about 100MHz would require a fiber immersion surface contact diameter of about 7μm for sampling at 100 MHz without introducing spatial averaging. It ispossible that the diameter of the fiber would be less than 20 um, oreven less than 30 um, and that the diameter may depend on the frequencyof the signal being measured. The immersion end of the fiber may beprovided with a thin layer of material having a thickness of from about2 nm to about 12 nm on the immersion surface contact of the fiber andmay also have an associated geometry, such as, for example, tapering,cleaving, or etching. As an example, the fiber optic hydrophonesdisclosed herein can be configured to measure the p-t waveform of theultrasound field. These measurements may be used in one or more ways,such as, for example, for calibration of ultrasound devices, as areference for ultrasound devices during use of the ultrasound device,metrology, medical metrology, determination of complex sensitivity vs.frequency, imaging, detection, sonar, diagnosis, communications, therapyand the like. Such devices may be optimized for sensitivity based onspecific wavelengths, geometries, indexes of refraction, composition,diameters, plasmon excitation frequencies, and the like, for theselected thin layer materials.

In an example embodiment, a light source, a detector, and a thin layerof material can be provided. Light from the light source may betransmitted through the fiber. The thin layer of material is depositedon the immersion surface contact of the fiber, wherein the immersion endof the fiber is contacted with an immersion material, wherein theimmersion material is any gas, liquid or solid. The immersion surfacecontact may thusly be contacted with the liquid, solid or gas. Thedetector may detect Fresnel back reflections due to the change in indexof refraction between the immersion contact surface of the fiber, thethin layer of material and the immersion material, or solid. Changes inpressure may cause the index of refraction of the immersion material orsolid to change, or it may place strain on the immersion material, as itmay also cause the index of refraction of the thin layer of material tochange due to strain. The changes in index cause a change in the Fresnelback reflections. The detector detects these changes in Fresnelreflections and correlates them with the change in index of refraction.Pressure measurements of the immersion medium or solid may then bedetermined. In an embodiment, light from the source passes through thefiber, is reflected from the end of the fiber by the thin film ofdielectric material and is detected by a detector. Variations in theamount of reflected light may be used to determine the pressure actingon the thin film, which may be proportionate to the pressure acting onthe immersion material or solid material.

In a configuration of the example embodiment mentioned above, thethickness of the thin film may be in the range of from about 1 nm toabout 30 nm. Early research in these thin films predicted that adecrease in the thickness of a thin film below 30 nm would not yield anincrease in the sensitivity. This view was due in part to the perceptionthat the thin layer was over an order of magnitude thinner than thewavelength of light (1550 nm wavelength with a 50 nm layer of material),so no great variation in sensitivity was expected. Surprisingly,however, new increases in sensitivity and sensitivity maxima were foundby decreasing the layer thickness. For example, in one configuration,the thickness of the film may be in the range of from about 1.5 nm toabout 25 nm. In another configuration, the thickness of the film may bein the range of from about 2 nm to about 20 nm. In a furtherconfiguration, the thickness of the film may be in the range of fromabout 2.5 nm to about 15 nm. In another configuration, the thickness ofthe film can be in the range of from about 3 nm to about 10 nm, or evenin the range of from about 3.5 nm to about 8 nm.

In an example embodiment, the optimum thickness of the thin film may bedetermined based on the optimum back reflectance equation:

$\frac{\partial R}{\partial p} = {{\frac{\partial R}{\partial n_{c}}\frac{\partial n_{c}}{\partial p}} + {\frac{\partial R}{\partial n_{w}}\frac{\partial n_{w}}{\partial p}} + {\frac{\partial R}{\partial n_{g}}\frac{\partial n_{g}}{\partial p}} + {j\frac{\partial R}{\partial k_{g}}\frac{\partial k_{g}}{\partial p}}}$where$R = {{\Gamma_{in}\Gamma_{in}*{and}\mspace{14mu}\Gamma_{in}} = \frac{{n_{d}\left( {n_{c} - n_{w}} \right)} + {\left( {{n_{w}n_{c}} - n_{d}^{2}} \right)\tanh\;\gamma\; d}}{{n_{d}\left( {n_{c} + n_{w}} \right)} + {\left( {{n_{w}n_{c}} + n_{d}^{2}} \right)\tanh\;\gamma\; d}}}$In such an embodiment, several factors may be influential, such as, forexample, γ=

${j\left( \frac{2{\pi\left( {n - {j \cdot k}} \right)}}{\lambda} \right)},$where λ the wavelength of the laser, the index of refraction of thefiber (n_(c)), the geometry of the fiber, the index of the immersionmaterial (n_(w)), and the type of material of the thin layer (n_(d)).The optimum change in compression of thin layer due to pressure may bedescribed as

$\frac{\partial R}{\partial p} = {\frac{\partial R}{\partial d}{\frac{\partial d}{\partial p}.}}$

Optimum slope value of reflectance and pressure is achieved in optimumchange in complex index of coating layer due to pressure. When theplasmon excitation frequency of the thin layer of material given in thefollowing equation matches with particular light source wavelength, thenan increase in complex index is observed and the like may be consideredin determining a desired thickness of the thin layer of material.

$\left( {{{{Re}\left\lbrack {ɛ(\omega)} \right\rbrack} = {1 - \frac{\omega_{p}^{2}}{\omega^{2}}}},} \right.$Where, ∈(ω) is the complex permittivity of the thin film, ω_(p) is thesurface plasmon resonance frequency.

In an embodiment, a fiber optic hydrophone disclosed herein operates inthe reflective mode based on the principle of intensity modulation inFresnel reflectance from the immersion surface contact of an opticalfiber. The fiber can be immersed in, or placed in contact with animmersion medium or solid, where an immersion medium can be any gas,liquid, or solid. Acoustic waves acting on the immersion medium maycause a change in the index of refraction of the immersion medium, whichin turn would cause a change in the Fresnel reflectance. Moreover,measurements in the change of Fresnel back reflectance may also becorrelated with changes in pressure at the immersed end. Other aspectsof the principle and use of reflective fiber optic hydrophones will bewell known to one having ordinary skill in the art.

In an embodiment, an addition of a thin layer of material on theimmersion surface contact of a fiber sensor can provide an improvementin sensitivity over the sensitivity of an uncoated fiber. Initialexploration of this phenomena using transmission line models and otheranalytical methods provided an estimate of ideal thickness on the orderof over about 30 nm. Surprisingly, however, improved models, testing,and methods have shown that there is a further increase in sensitivityusing thinner layers than previously suggested. In fact, using thepreviously known models, any decrease from a 30 nm coating layer wouldnot have provided any increase in the sensitivity of the hydrophone. Newexperiments and analysis have shown, however that there is a surprisingincrease in sensitivity with even thinner layers of material.

In another embodiment a wavelength of a light source or laser, the taperangle of the immersion end of the fiber, the core and cladding diameter,taper length, fiber index, thin layer material, thin layer thickness andother fiber geometries may be varied in order to change, and in somecircumstances to increase the sensitivity of the fiber optic hydrophone.As an example, the factors listed above may be adapted to optimize thesensitivity of a fiber in a particular circumstance and for a particularuse. In another embodiment, the plasmon excitation frequency of thematerial selected for use as the thin layer material may be consideredin determining the ideal thickness of the layer. For example the plasmonresonance frequency for a given material at a given wavelength may beused to determine the optimal thickness for the thin layer. As a furtherexample, a method of manufacture of these materials may optimize a fiberoptic hydrophone by varying the above noted elements for a particularuse or a range of uses. These uses may include, without limitation,metrology, medical metrology, determination of elasticity, of ultrasoundor other acoustic devices, calibration of medical instruments,diagnosis, therapy, other medical uses, sonar, communications and thelike.

FIG. 1 is a depiction for the setup of a previously known fiber optichydrophone that operates in a reflective mode. In FIG. 1, there is alight source 100 which provides light to a fiber 104, where the fiberhas a diameter at the immersion surface contact of greater than 15 μm.Light passes through the fiber which has one end of the fiber 110immersed in a medium or contacted with a solid 108. Some portion of thelight from the source 100 can be reflected from the fiber/mediuminterface 112, which can be transported back through the fiber 104,split off by beam splitter 106 and detected by detector 102. Variationsin pressure in the immersion medium or solid 108 can cause changes inthe index of reflection of the immersion medium or solid, 108, which canin turn cause a change in the amount of light reflected back to detector102. Thus, a measurement of pressure may be made based on the amount oflight received by detector 102.

FIG. 2 depicts a non-limiting experimental setup of an exampleembodiment of the present invention. One having ordinary skill in theart would understand that certain elements of the diagram are notnecessary to the operation of the fiber optic hydrophone, that anynumber of elements may be added to the setup and that the fiber optichydrophone can vary in a number of ways, including ordering of specificelements, from the experimental setup disclosed in FIG. 2.

A fiber optic hydrophone may comprise a light source 202. The lightsource depicted in FIG. 2 may be any light source capable of providinglight to a fiber and a Fresnel back reflection from the interfacebetween the immersion end of the fiber and the immersion material orsolid. In one embodiment, the light source 202 may be any type of laser,distributed feed back laser, pulsed laser, continuous wave laser, lightemitting diode, bulb, laser diode, lamp, bulb or the like. In a seriesof non-limiting example embodiments, the light source may be one or morelasers or LEDs with an output anywhere in the range of about 300 nm toabout 2500 nm, or, as a series of non-limiting examples, sources havingwavelengths in the range of from about 450 nm to about 900 nm, or in therange of from about 900 nm to about 1300 nm, or of from about 1300 nm toabout 1700 nm. The light source may also emit radiation at about 1620nm, 1550 nm, 1410 nm, 1060 nm, 980 nm, 850 nm, 830 nm, 800 nm, 780 nm,680 nm, or 633 nm. The light source emits light that is coupled into thefiber optic 228 for Fresnel back reflections that may be detected by thedetector 216 as show above with respect to FIG. 2. The light source maybe polarized, or it may be unpolarized.

In an embodiment depicted in FIG. 2, a fiber optic hydrophone maycomprise one or more of optical isolators 204 and 210, optical couplers,206 and 211, and amplifiers such as 208. As a series of exampleembodiments, an optical amplifier can be an erbium doped amplifier, or,as another example, amplifiers may be selected such that they amplifywithin a range of emission from light source 202. Further, the isolators204 and 210 and couplers 206 and 211 may also be selected such that theyisolate and couple one or more wavelengths of light within a range ofemission from light source 202. While FIG. 2 depicts couplers, amplifierand isolators, there is no requirement that a fiber optic hydrophonehave any of these elements, though a fiber optic hydrophone may have oneor more of any of the above.

In an embodiment, a signal generator 212, power amplifier 214, andultrasound generator 226 may provide a signal for an output that may becaptured by the immersion end of fiber optic 228. While the figuredepicts an experimental setup, wherein the signal generator 212, poweramplifier 214, and ultrasound generator 226 are used to test thesensitivity and durability of fiber optic hydrophone 200, in anotherembodiment, a signal generator and amplifier may be configured totransmit through, or reflect pressure amplitude off of, one or moresurfaces. Transmissions or reflections off of these surfaces could beused by one or more fiber optic hydrophones to determine aspects of thesurfaces. As an example, signal generator 212, power amplifier 214, andultrasound generator 226 could be used to provide a signal which couldreflect off of one or more organs of a patient. These reflected signalscould be detected by an array of fiber optic hydrophones such as fiberoptic hydrophone 200 to image the organs, cells, tissue and the like. Itshould be noted that while reflecting signals off of surfaces is one wayto image surfaces, this is not intended to be a limiting example, andone or more fiber optic hydrophones could be used to image surfaces inany manner known in the art.

As another example, a communications system may be created using thefiber optics hydrophones disclosed herein. For example, in a navalvessel or army tank having a closed compartment where it is desirous notto have wires entering or exiting the compartment, pressure amplitudewith a high frequency bandwidth may be transmitted by one or more signalgenerators and detected by a fiber optic hydrophone after the signalpasses through a structure. The high bandwidth possibilities in a rangeof up to about 200 MHz may be detected without error and such highbandwidth may provide high speed communications. Further, an array offiber optic hydrophones could be used in other configurations todetermine the wavefront of an acoustic wave over an area.

Signal generator 212, power amplifier 214, and ultrasound generator 226show one way to create acoustic signals for imaging or communications orthe like. Any other type of acoustic signal generator may also be usedincluding one or more of naturally occurring, mechanical, or electricsignal generators.

FIG. 2 depicts a detector 216 and a spectrum analyzer 218. The detector216 may be any detector known in the art capable of detectingelectromagnetic radiation. In one embodiment, an avalanche photodiode, aPIN photodetector, may be used. In another embodiment, a CCD, or a APDdetector is used. Any other detector may be selected based onsensitivity requirements and the wavelength and/or wavelengths of lightemitted from the light source and transmitted through the fiber. Thedetector 216 can detect changes in Fresnel back reflections from theimmersion end of fiber optic 228.

FIG. 2 depicts a water tank 224. The water tank was used as anexperimental setup, however, the fiber may be immersed in any immersionmaterial or contacted with a solid. As an example, water tank 224 couldrepresent an individual, wherein the fiber optic hydrophone 200 is usedin vivo during an operation. As another example, the fiber optichydrophone could be placed on a surface, such as the skin of a patientor animal. As another embodiment, a medium could be placed in contactwith an item of interest and fiber optic hydrophone 200 can be placed inthe gas or fluid such that it may determine aspects of the pressureamplitude of the item. As a further example, the immersion materialcould be any gas, or any liquid, such as, for example, air or saltwater.

FIG. 2 depicts examples of two immersion ends of fiber optic 228.Immersion end 222 an uncoated tapered fiber and 220 is tapered fiberwith a layer of gold on the immersion surface contact of the fiberoptic. In the experimental setup of FIG. 2, each fiber has a tapered endand there is an uncoated end on one of the fibers. These arenon-limiting embodiments of the immersion end of the fiber. In anotherembodiment, the immersion end of the fiber comprises an untapered endand a thin layer of material on the immersion contact surface of thefiber. As another embodiment, the immersion end of the fiber maycomprise a tapered end and an angle cleaved immersion contact surfacecoated with a layer of material. The taper angle, cleave angle, andtaper length may vary in any way known in the art. These aspects of theimmersion end of the fiber may be selected such that the sensitivity ofthe fiber optic hydrophone is maximized for a particular use or for arange of uses.

In an embodiment, the fiber optic 228 of FIG. 2 can be selected suchthat it will transmit light emitted by light source 202 depicted withrespect to FIG. 2. This does not mean to imply that the fiber may onlybe selected based on the light source, or that the fiber must beoptimized for transmission of the light source, however, in oneembodiment, fibers optimized for light source outputs 202 may be used inthe fiber optic hydrophone. In a further embodiment, the index ofrefraction of the fiber optic is provided and may be used in one or moreways to determine thicknesses of the thin layer of material as depictedabove with respect to FIG. 2. For example, the intensity of a Fresnelreflection may be varied based on the index of refraction of the fiber.

In an embodiment, the fiber 228 may be a single mode fiber and may havea core diameter at the immersion surface contact of less than about 20um, 15 μm, or even less than about 10 μm. As a further example, the corediameter at the immersion surface contact may be in the range of fromabout 2.5 μm to about 12 μm, or in the range of from about 4 μm to about10 μm, or even in the range of from about 4.5 μm to about 8 μm. Fibershaving diameters at the immersion surface contact in the range above mayhave tapered ends, where the taper terminates with a fiber having corediameters in the above noted ranges. These fibers may also be etchedwhere the diameter of the core terminates at the immersion surfacecontact with a diameter in any of the ranges noted above. In oneembodiment the fibers may have cylindrical cores or elliptical cores orany other core geometry known in the art.

In one embodiment, selection of the diameter of the fiber is determinedbased on the frequency range of interest. For example, the active sensoraperture size, or fiber core diameter at the immersion surface contactmay be selected such that it is comparable with the half acousticwavelength at the highest frequency of interest. In such an example, inorder to eliminate the effects of spatial averaging, a hydrophone maysample a field with at least half-wavelength resolution in a range offrom about at least 70 MHz, to about 500 MHz, where, as an example,sampling a field with at least half-wavelength resolution of 100 MHz inwater would require an active sensor aperture size on the order of about7 μm. In other embodiments, the field of sampling may be up to about 500MHz. Based on these aperture requirements, single mode fibers may beselected for fiber optic hydrophones. The fiber 228 used in the fiberoptic hydrophones described herein may be any one of the fibersavailable for sale, or any other fibers known in the art.

As one example, the core of the fiber 228 should be selected such thatit approximates a point source for detection and is thus capable ofreceiving a large bandwidth of acoustic information. As noted above,several prior art technologies lacked the capability to appropriatelymeasure pressure amplitude without error due to spatial problems.Limiting the diameter of the core of the fiber to a single mode with anarrow diameter may provide a reduction in the spatial errors of thesystem.

Other elements may be included in the system. For example, in oneembodiment the system may further comprise one or more of opticalisolators, couplers, filters, positioners, lenses, polarizers, waveplates, mirrors, transducers, spectrum analyzers, additional lasers,amplifiers, signal analyzers and generators, power sources, detectors,fibers, positioning elements such as stands, mounts, robotic arms andthe like.

In addition, one or more computing elements may be associated with oneor more of the elements depicted in FIG. 2 and may provide input tothose elements and receive output. Computing elements may also performcalculations based on received input. For example, data output from thedetector and data output from the light source may be used to calculateor otherwise determine changes in pressure. In addition, any otherelement depicted in FIG. 2 or described above may be configured to workin conjunction with computing elements and may have associatedtherewith, output and input. These connections to one or more computingelements may be wired or wireless.

In one embodiment, the thin layer of material on the immersion surfacecontact of fiber 228 may comprise gold. Other metallic materials mayalso be used as the layer at the immersion surface contact of the fiber.As non-limiting examples, the layer may comprise one or more of Ag, Au,Pt, Al, Co, Ni, Ti, Ga, In, Cr, Mo, any other metal and/or any metallicalloy. Plasmon resonance frequencies of metallic materials having largechange in complex index of refraction properties during stress may beused to select a material. A metal or combination of metal materials maybe used to form the thin layer of material on the immersion surfacecontact of the fiber. In another embodiment, multiple layers may beprovided on the immersion surface contact of the fiber. As one example,the strain properties of a metal can be related to change of index ofrefraction of the metal. Knowledge of the plasmon excitation frequency,the reflectance and the strain properties of a metallic material or, inanother embodiment, a dielectric material may be used to optimize thesensitivity of a fiber optic hydrophone at a given wavelength for agiven application or range of applications.

In an embodiment, the thickness of the thin layer of material may beselected based on the plasmon excitation resonance frequency of thematerial for a given wavelength of light. Correlating the wavelengthwith the plasmon excitation frequency may provide an optimal thickness.As one example, the formula for selecting thickness based on thesefactors may be maximizing back reflectance change due to thicknessvariation and large change in thickness due to stress such as

$\frac{\partial R}{\partial p} = {\frac{\partial R}{\partial d}{\frac{\partial d}{\partial p}.}}$

In another embodiment, one or more dielectrics may be used as the thinlayer of material on the immersion surface contact of the fiber. Forexample, TiO2, may be used individually or in combination with any ofthe other metals and/or an dielectric material. As one example, thestrain properties of a material can be related to change of index ofrefraction of a material due to strain. Knowledge of the reflectance andstrain properties of a metallic material or dielectric material may beused to optimize the sensitivity of a fiber optic hydrophone for a givenapplication or range of applications.

The thin layer of material described above may be coated on theimmersion surface contact of the fiber using any method known in theart. As one example, a thin material such as, for example gold may becoated using one or more of sputtering techniques, molecular beamepitaxy, metaorganic chemical vapor deposition, evaporation techniques,wet or dry chemistry techniques, or any other film growth or coatingtechnique. In one embodiment, the fiber optic hydrophones disclosedherein may be placed in harsh environments, that is, environments havingvery high levels of pressure amplitude. Other known hydrophones may havefailed, for example, near or in the focal zones or volumes of HighIntensity Focused Ultrasound (HIFU) fields, High Intensity TherapeuticUltrasound (HITU) fields, or in Lithotripsy calibration. In oneembodiment, the fiber optic hydrophone disclosed herein may beparticularly suited to measurement of these high levels of pressureamplitude because a failure mode is peeling off of the thin layer ofmaterial on the tip of the fiber. In such an example, one or more of theabove mentioned layer growth or deposition methods may provide increasedadhesion.

In an embodiment the thickness of the layers can be selected such thatthey are optimized for sensitivity to pressure. As used herein,sensitivity to pressure may comprise the ratio of change in index ofrefraction of the layer to a change in pressure of the immersion mediumor solid. In one embodiment, the more a layer changes index ofrefraction as changes in pressure in an immersion medium or solid, themore sensitive it is. As an example of the above, changes in index ofrefraction may cause changes in the amount of light received at thedetector, which may thus be proportional to the change in pressure ofthe immersion medium or solid.

FIG. 3 depicts a series of immersion ends of a fiber optic hydrophone,where the immersion ends are those ends immersed in a medium, orcontacted with a solid. In one embodiment, when an immersion end of afiber is immersed in an immersion medium or contacted with a solid andit may detect pressure amplitude and changes in pressure. As notedabove, a fiber may be a straight cleaved fiber 302 where there is noetching or tapering of the fiber. The straight cleaved fiber will havean immersion surface contact 304 and, in one embodiment, comprise a thinlayer of material 306.

In an embodiment, the immersion end of a fiber optic hydrophone may alsobe a tapered immersion end 308. The tapered fiber may have a taperlength 310 and a taper angle 312. The taper may be linear, or it may beexponential or it may have any other suitable geometry such as parabolicor hyperbolic. The tapered fiber will have an immersion surface contact304 and may comprise a thin layer of material 306.

In an embodiment, the immersion end of a fiber optic hydrophone may alsobe an etched end 314. In one embodiment this will be a cylindrical etch,while in another embodiment it may be an elliptical etch or have adifferent etching geometry. The immersion end of the fiber will have animmersion surface contact 304 and may comprise a thin layer of material306.

In another embodiment, the immersion end of a fiber optic hydrophone mayhave be an angle cleaved fiber 318. This cleave angle 320 may be anyangle in a range of from about 0 degrees to about 45 degrees. Theimmersion contact surface of the angled cleave may comprise a thin layerof material 306.

FIGS. 4(a)-(f) depict a series of images and modeled geometry for: (a)and (b) a straight cleaved uncoated fiber optic hydrophone; (c) and (d),a cylindrically etched fiber optic hydrophone and (e) and (f) a linearlytapered fiber optic hydrophone. Each of the sensors may comprise a thinlayer of material on the end as noted above with respect to FIG. 2. Asnoted with respect to FIG. 4, the fiber may be cylindrical, orcylindrically etched. Further, the fiber may be tapered as shown withrespect to FIG. 4(e). In one embodiment the fibers may have cylindricalcores or elliptical cores, and may be either cylindrically etched orelliptically etched, or etched otherwise.

The tapering of the fiber may be linear, however, in another embodiment,the tapering may not be linear, but may instead be an exponentialtapering or it may be tapered according to any other method known in theart, such as, for example, hyperbolic or parabolic or otherwisenon-linear. As an example, the tapering angle may be in a range ofbetween about 0 degrees and about 45 degrees. The tapering length on afiber optic hydrophone may be any length known in the art and maycomprise any length in a range of from about 10 μm to about 1 mm.

The selection of the tapering angle, tapering length and othergeometries of the tip of the fiber may be selected in order to optimizethe sensitivity of the fiber optic hydrophone. As shown below withrespect to FIG. 7, in one experimental setup, the taper angle andgeometry had an effect on the sensitivity. While the taper angle andtaper length may be beneficial to the sensitivity of the fiber optichydrophone in some embodiments, the above is not intended to imply thattaper angles and lengths are necessary for any particular embodiment.Further, it should be noted that various aspects of a fiber optichydrophone have relationships between them. For example, a straightcleaved, non-etched fiber at 980 nm require a thin layer of material ata first thickness in order to optimize the sensitivity of the fiberoptic hydrophone. However, a tapered, etched fiber at 980 nm may requirea thin layer of material at a second thickness in order to optimize thesensitivity of the fiber optic hydrophone.

FIGS. 4(a) and (b) depict a fiber having a core diameter 402 and anun-etched cladding diameter 404. As noted above, the core diameter ofthe fiber may be selected such that it approximates a point source fordetection and is thus capable of receiving a large bandwidth of acousticinformation. As noted above, several prior art technologies lacked thecapability to appropriately measure pressure amplitude without error dueto spatial problems. Limiting the diameter of the core of the fiber to asingle mode with a narrow diameter may provide a reduction in thespatial errors of the system. In an embodiment, the diameter of thefiber core is less than 8 μm. Elements 406 and 408 depict the diameterof the cladding and core of an etched fiber, and 412 and 414 depict thediameter of a tapered fiber.

FIG. 5 depicts a logarithmic graph with the expected results for a thinlayer of gold placed on the immersion surface contact of a straightcleaved fiber optic hydrophone. For this graph, classical approaches ofmodeling were used and it can be seen that the optimal thickness wasgreater than 30 nm.

FIG. 6 depicts surprisingly good experimental results in sensitivity ofa fiber optic hydrophone at 1550 nm with a thin layer of gold coated onthe immersion surface contact of the immersion end of a fiber. Thediameter of the fiber in this example was 6 μm, and the fiber wascylindrically etched. As is clear from comparing FIG. 5 and FIG. 6,there is a considerable difference in the expected peak using thinnermaterials. FIG. 6 depicts a single setup of a fiber optic hydrophone andis not meant to depict the optimal thickness of a thin layer on theimmersion contact surface for all fiber optic hydrophones. As notedabove, altering one or more aspects of the setup, including thewavelength, index of refraction, thin layer material, fiber optic,immersion material and geometry of the fiber can affect the optimalthickness.

FIG. 7(a) depicts an example of the effect of taper angel on sensitivityfor linearly tapered angles (dotted line) and exponentially taperedangles (solid line). The vertical axis of the graph represents anexample improvement in sensitivity in dB. FIG. 7(b) depicts the effectof fiber diameter of a cylindrically etched fiber on sensitivity. Thevertical axis of the graph represents an example improvement insensitivity in dB. FIG. 7 provides one example of some potential effectsfrom alterations in geometry to the immersion end of a fiber optichydrophone.

As noted above, it is expected that the fiber optic hydrophonesdescribed may be manufactured or used in a variety of manners. As afirst example, fiber optic hydrophones may be manufactured optimized fora particular use. Uses include calibration of medical devices. In oneexample of a calibration, the fiber optic hydrophone disclosed hereinmay be placed near or in the focal zones of High Intensity FocusedUltrasound (HIFU) fields, High Intensity Therapeutic Ultrasound (HITU)fields, or Lithotripsy devices. The fiber optic hydrophone may be usedto calibrate any device used to create HIFU or HITU fields, as well asany other medical ultrasound device, including those used for ultrasoundimaging. As another example use, an array of fiber optic hydrophonescould be used for imaging. The array may detect signals in a reflectiveor transmissive mode from a source and may image one or more objectsbased on the reflections. As another example, the fiber optic hydrophonemay be used in sonar applications, to detect changes in pressure inwater or to image objects in media. As another example, detection ofpressure amplitude may be used in theragnostics, diagnosis, treatmentand therapy of a broad range of issues in humans or animals. In oneconfiguration, the hydrophones may be used in situ. In another example,the hydrophones may be used in vivo and in another application, thefiber optic hydrophones described herein may be used in an in vitroapplication.

The device describe herein may be used as a single point fiber optichydrophone for characterization of one or more acoustic signals. Asanother example, an array of fiber optic hydrophones described abovewith respect to FIGS. 2-4 may be provided. The array could be, forexample, a linear array comprising two or more fiber optic hydrophones.As another example, a two dimensional array could be created out ofthree or more hydrophones. For example, fiber optic hydrophones could beplaced in a circular array, a triangular array, a square, a sparsearray, a rectangular array or in an amorphous array. Such an array couldbe used to image or to characterize one or more aspects of an acousticwavefront in an immersion contact medium or a solid.

In an embodiment, there is provided a system for detecting changes inpressure in an immersion medium or solid, the system comprising: anoptical fiber, wherein said optical fiber has a core diameter at animmersion surface contact of the fiber of less than 10 μm; and a layerof material deposited on the immersion surface contact of the fiber,wherein said layer of material is of a thickness of from about 2 nm toabout 10 nm.

In an alternate embodiment, the system may further comprise a lightsource and a detector.

In an alternate embodiment, the system may further comprise a computerprogrammed to relate Fresnel back reflections from an immersion end ofthe fiber to said changes in pressure.

In an alternate embodiment, the system may comprise the layer ofmaterial is a layer of gold.

In an alternate embodiment, the system may comprise the layer ofmaterial is a layer comprising one or more metals.

In an alternate embodiment, the system may comprise the layer ofmaterial is a layer comprising one or more dielectrics.

In an alternate embodiment, the system may comprise the layer ofmaterial is a layer comprising one or more of a metal and a dielectric.

In an alternate embodiment, the system may comprise the layer ofmaterial is a layer comprising one or more of Ag, Au, Pt, and TiO2.

In an alternate embodiment, the system may comprise the light sourcecomprises a laser having a wavelength output within a range of fromabout 500 nm to about 1700 nm.

In an alternate embodiment, the system may comprise the light sourcecomprises a laser having a wavelength output within a range of fromabout 1500 nm to about 1600 nm.

In an alternate embodiment, the system may comprise the light sourcecomprises a laser having a wavelength output within a range of fromabout 900 nm to about 1300 nm.

In an alternate embodiment, the system may comprise the thickness of thelayer of material is of from about 3.5 nm to about 8 nm.

In an alternate embodiment, the system may comprise an immersion end ofsaid fiber comprises a taper angle and a taper length.

In an alternate embodiment, the system may comprise the taper angle isof from about 0 to about 45, and wherein the taper length is of fromabout 10 μm to about 1 mm.

In an alternate embodiment, the system may comprise the diameter of thecore of the fiber at the immersion surface contact is from about 4.5 μmto about 8 μm.

In an alternate embodiment, the system may comprise said detector isconfigured to detect light back reflected from said thin layer ofmaterial and wherein said changes in the reflectance of said thin layerof material are measured by said detector.

In an embodiment, there is provided a method for detecting pressurewaves in an immersion medium or solid, the method comprising: contactingthe immersion medium or solid with a fiber optic, wherein said fiberoptic is integrated with a light source and a detector, and wherein saidfiber optic has a diameter of less than 10 μm at an immersion surfacecontact of the fiber; providing a thin layer of material on theimmersion surface contact, wherein said thin layer of material is of athickness in a range of from about 2 nm to about 10 nm; and detectingFresnel back reflections from an immersion end of the fiber.

In an alternate embodiment, the method comprises contacting theimmersion surface contact with the immersion material or solid.

In an alternate embodiment, the method comprises the Fresnel backreflections are related to acoustic pressure differences in said medium.

In an alternate embodiment, the method comprises said layer of materialon the immersion surface contact is of a thickness of from about 3.5 nmto about 8 nm.

In an alternate embodiment, the method comprises said fiber has adiameter at the immersion surface contact in a range of from about 4.5μm to about 8 μm.

In an alternate embodiment, the method comprises said fiber hasassociated therewith, a taper angle and a taper length.

In an alternate embodiment, the method comprises wherein said taperangle is in a range of from about 0 degrees to about 45 degrees andwherein said taper length is in range of from about 10 μm to about 1 mm.

In an alternate embodiment, the method comprises the light sourcecomprises a laser having a wavelength output within a range of fromabout 300 nm to about 1700 nm.

In an alternate embodiment, the method comprises the light sourcecomprises a laser having a wavelength output within a range of fromabout 1500 nm to about 1600 nm.

In an alternate embodiment, the method comprises the light sourcecomprises a laser having a wavelength output within a range of fromabout 900 nm to about 1100 nm.

In an alternate embodiment, the method comprises the layer of materialis a layer of gold.

In an alternate embodiment, the method comprises the layer of materialis a layer comprising one or more metal.

In an alternate embodiment, the method comprises the layer of materialis a layer comprising one or more dielectric compound.

In an alternate embodiment, the method comprises the layer of materialis a layer comprising one or more of a metal and a dielectric.

In an alternate embodiment, the method comprises the layer of materialis a layer comprising one or more of Ag, Au, Pt, and TiO2.

In an embodiment, there is provided a method of making a device fordetecting acoustic waves in medium comprising: providing an opticalfiber having a core diameter at an immersion surface contact of lessthan about 10 μm; and depositing on said immersion surface contact alayer of material having a thickness of from about 2 nm to about 10 nm.

In an alternate embodiment, the method of making a device furthercomprises integrating the optical fiber with a light source and adetector.

In an alternate embodiment, the method of making a device furthercomprises the layer has a thickness of between about 3.5 nm to about 8nm.

In an alternate embodiment, the method of making a device furthercomprises the fiber has a core diameter at the immersion surface contactin the range of from about 4.5 μm to about 8 μm.

In an alternate embodiment, the method of making a device furthercomprises said thin layer of material is selected based on the formula

$\frac{\partial R}{\partial p} = {\frac{\partial R}{\partial d}{\frac{\partial d}{\partial p}.}}$

In an alternate embodiment, the method of making a device furthercomprises said thin layer of material comprises a metal layer.

In an alternate embodiment, the method of making a device furthercomprises said thin layer of material comprises a dielectric.

In an alternate embodiment, the method of making a device furthercomprises the layer of material comprises gold.

In an alternate embodiment, the method of making a device furthercomprises said diameter of said fiber at the immersion surface contactof the fiber is selected for single point spatial sampling for abandwidth of at least 100 MHz.

In an alternate embodiment, the method of making a device furthercomprises the light source comprises a laser having a wavelength outputwithin a range of from about 300 nm to about 1700 nm.

In an alternate embodiment, the method of making a device furthercomprises the light source comprises a laser having a wavelength outputwithin a range of from about 900 nm to about 1100 nm.

In an alternate embodiment, the method of making a device furthercomprises the light source comprises a laser having a wavelength outputwithin a range of from about 1450 nm to about 1600 nm.

In an alternate embodiment, the method of making a device furthercomprises said layer of material is selected based on sensitivity of thecomplex index of refraction of said layer of material to changes inpressure acting on the layer of material.

In an alternate embodiment, the method of making a device furthercomprises configuring two or more devices in a linear array.

In an alternate embodiment, the method of making a device furthercomprises configuring two or more devices in a two dimensional array.

It should be understood that the configurations and/or approachesdescribed herein are exemplary in nature, and that these specificembodiments or examples are not to be considered limiting. The specificroutines or methods described herein may represent one or more of anynumber of strategies. As such, various acts illustrated may be performedin the sequence illustrated, in other sequences, in parallel, or thelike. Likewise, the order of the above-described processes may bechanged.

Additionally, the subject matter of the present disclosure includescombinations and sub-combinations of the various processes, systems andconfigurations, and other features, functions, acts, and/or propertiesdisclosed herein, as well as equivalents thereof.

What is claimed:
 1. A method for detecting pressure waves in animmersion medium or solid, the method comprising: contacting theimmersion medium or solid with a fiber optic having an immersion surfacecontact region and a thin layer of material surmounting an immersionsurface contact region end, wherein the thin layer of material on theimmersion surface contact region is of a thickness of from about 3.5 nmto 6 nm, wherein using the thickness of the thin layer of the materialwithin the range from about 3.5 nm to 6 nm improves sensitivity of thedetection over a fiber optic with no layer of material surmounting animmersion surface of the contact region by over 35 dB, said fiber opticbeing integrated with a light source and a detector, said fiber optichaving a diameter of less than 30 um at an immersion surface contact ofthe fiber; and detecting Fresnel back reflections from an immersion endof the fiber.
 2. The method of claim 1 wherein said fiber has associatedtherewith, a taper angle and a taper length.
 3. The method of claim 2wherein said taper angle is in a range of from about 10 degrees to about45 degrees and wherein said taper length is in range of from about 10 umto about 1 mm.
 4. The method of claim 1 wherein the layer of material isa layer comprising one or more dielectric compounds.
 5. The method ofclaim 1 wherein the layer of material is a layer comprising one or moreof a metal and a dielectric.
 6. The method of claim 1 wherein the layerof material is a layer comprising one or more of Ag, Pt, and TiO2. 7.The method of claim 1, wherein a diameter at the immersion surface ofthe fiber core is less than 10 μm.
 8. The method of claim 1, wherein theimmersion surface contact region end has an etched end shape.
 9. Themethod of claim 1, wherein the immersion surface contact region end hasa cleaved end shape.
 10. A system for detecting changes in pressure inan immersion medium or solid, the system comprising: an optical fiberhaving a layer of material deposited on an immersion surface contactregion of the fiber, wherein the layer of material on the immersionsurface contact region is of a thickness of from about 3.5 nm to 6 nm,wherein using the thickness of the thin layer of the material within therange from about 3.5 nm to 6 nm improves sensitivity of the detectionover a fiber optic with no layer of material surmounting an immersionsurface of the contact region by over 35 dB, wherein the optical fiberhas a core diameter at the immersion surface contact region of the fiberof less than about 30 um.
 11. The system of claim 10, wherein the layerof material is a layer comprising one or more of a metal and adielectric.
 12. The system of claim 11, wherein the layer of material isa layer comprising one or more of Ag, Pt, and TiO2.
 13. The system ofclaim 10, wherein an immersion end of said fiber comprises a taper angleand a taper length.
 14. The system of claim 13, wherein the taper angleis of from about 10 to about 45 degrees, and wherein the taper length isof from about 10 um to about 1 mm.
 15. The system of claim 10, whereinthe system includes a detector that is configured to detect light backreflected from said thin layer of material and wherein said changes inthe reflectance of said thin layer of material are measured by thedetector.
 16. The system of claim 10, wherein a diameter at theimmersion surface of the fiber core is less than 10 μm.
 17. The systemof claim 10, wherein the immersion surface contact region end has anetched end shape.
 18. The system of claim 10, wherein the immersionsurface contact region end has a cleaved end shape.